Immobilization
of titanium dioxide nanoparticles (TiO<sub>2</sub> NPs) facilitates their removal
and reuse in water treatment applications. Composite materials of
electrostatically-bound TiO<sub>2</sub> NPs and zeolite particles have been
proposed, but limited mechanistic studies are available on their performance in
complex media. This study delineates the relative importance of homo- and
heteroaggregation, water chemistry, and surface fouling by natural organic
matter (NOM) on the photocatalytic degradation of diethyltoluamide (DEET) by
TiO<sub>2</sub>-zeolite composites. Zeolite adsorbs a portion of the DEET,
rendering it unavailable for degradation; corrections for this adsorption
depletion allowed appropriate comparison of the reactivity of the composites to
the NPs alone. The TiO<sub>2</sub>-zeolite composites showed enhanced DEET degradation
in moderately hard water (MHW) compared to deionized water (DIW), likely
attributable to the influence of HCO<sub>3</sub><sup>−</sup>, whereas a net decline in reactivity was observed
for the TiO<sub>2</sub> NPs alone upon homoaggregation in MHW. The composites also
better maintained reactivity in the presence of NOM in MHW, as removal of Ca<sup>2+</sup>
onto the zeolite mitigated fouling of the TiO<sub>2</sub> surface by NOM. However,
NOM induced partial dissociation of the composites. DEET byproduct formation, identified
by quadrupole–time of flight (QTOF) mass spectrometry, was generally unaffected
by the zeolite, while NOM fouling favored de-ethylation over hydroxylation
products. Overall, the most significant factor influencing TiO<sub>2</sub>
reactivity toward DEET was NOM adsorption, followed by homoaggregation, electrolytes
(here, MHW versus DIW), and heteroaggregation. These findings can inform a
better understanding of NP reactivity in engineered water treatment applications.
DEET Degradation by Titanium Dioxide–Zeolite Nanocomposites: Effects of Aggregation State, Water Chemistry, and Natural Organic Matter